Measuring and control method and apparatus



Dec. 26, 1950 Filed larch 31. 1944 T. R. HARRISON ETAL IEASURING AND CONTROL METHOD AND APPARATUS 4 Sheets-shat 1 =F H2) '0 K|O| 5 91 nvmvrox s 9e THOMAS R.HARRISON' JF? LLOYD B. CHERRY Dec. 26, 1950 'r. R. HARRISON :rm. I 2,535,412

MEASURING AND CONTROL METHOD AND APPARATUS Filed larch 31, 1944 4 Shuts-Sheet 2 FIG. 2

INVENTOR. THOMAS R. HARRISON Y LLOYD B. CHERRY ATTOR Dec. 26, 1950 T. R. HARRISON ErAL 2,535,412

MEASURING AND CONTROL METHOD AND APPARATUS Filed larch 31, 1944 4 Sheets-Sheet 3 P D FIG. 5 A

o B Ill :5; z 3 Q 4 /4 '6" 'i 2 3 E, D.

0 500 I000 I500 I700 ssemu. m mcRovou's 1 FIG. 6 G

, INVENTOR.

THOMAS R.HARRISON Y BY LLOYD B.CHERRYY ATTORzY.

Filed larch 51, 1944 Dec. 26, 1950 T. R. HARRISON -'T AL 2,535,412;

umstmme AND coumox. ms'ruon AND APPARATUS 4 Sheets-Sheet. 4

,FIG. 7 32 on'uunu v 92 96 55 I02 M 'R-L m ul? F? i n? INVENTOR. THOMAS R. HARRISON LLOYD B. CHERRY ATTOR Patented Dec. 26, 1950 2,535,412 MEASURING AND CONTROL METHOD AND I APPARATUS Thomas R. Harrison, Wyncote. and Lloyd B. Cherry, Philadelphia County, Pa., assignors, by mesne assignments, to Minneapolis-Honeywell Regulator Company,

corporation of Delaware Application March 31, 1944, Serial No. 528,866

The present invention relates to an improved method of and apparatus for detecting and measuring magnetic flux. V

A general object of the invention is to provide an improved method of and apparatus for de-' tecting and measuring the total number of lines 26 Claims. (Cl. 175-183) Minneapolis,

Minn., a

maybe made to vary in a linear manner over a wide range with the magnitude of electromotive force obtained from an exploring object, such of induction in a magnetic field which are cut by an exploring object subjected to said magnetic field.

It is also an object of the invention to provide an improved method of and apparatus of this character in which such detection and measurement are independent of the rate of travel of the as a coil comprising a suitable number of turns, which is subjected to a magnetic field under observation. Such linearity of response is obtained over a range of approximately 1700 microvolts, motor rotation being initiated by a very small electromotive' force of approximately microvolts, by means of the present invention notwithstanding the fact that prior art motor drive systems capable of initiating motor operation on 10 microvolts produce maximum motor speed with coil through the magnetic field or. conversely'the rate of travel of the magnetic field past the coil.

Another object of the invention is to provide such an improved method and apparatus which provide an indication oi. the relative directions of movement of the magnetic field and the exploring object.

A specific object of the invention is to provide improvements in apparatus for indicating and /or recording the presence. of magnetic flux in the vicinity of an exploring .coil and also the relative-directions of movement of the magnetic field and said exploring coil. A more specific object of the invention is to provide such improved apparatus in which control means are incorporated for actuating an audible or visual signal when the total number of lines oi.' induction cutby the exploring coil exceeds a predetermined value.

an input electromotive force of microvolts or less. This desirable operation is obtained by superimposing an opposing electromotive force on that derived from the exploring coil to suppress the influence of the latter in tending to increase the motor speed as the derived electromotive force increases in magnitude. In this mannerthe motor speed increasing energizing efiect produced by the-electromotive force derived from the exploring coil may be suppressed to such an extent that the motor does not reach full speed until the exploring coil electromotive force attains a value of. approximately 1700 microvolts even though the motor would otherwise reach full speed when an electromotive force It is a particular object of the invention to' provide an improved indicating and/or recording apparatus. for indicating and/or recording the total number of lines of induction in a magnetic field which are cut by an exploring object wherein the indicating and/or recording elements are positioned by power-set means including a rugged reversible electrical motor. v

A further specific object of the invention is to provide such indicating and/or recording apparatus wherein suitable means are incorporated to returnthe indicating and/or recording elements to a predetermined central position and to there maintain the said elements as long as no. lines of induction are out by the exploring coil. f

of 30 microvolts or less is impressed on the input circuit of the motor drive system. Such suppression is accomplished, moreover. while retaining the desirable characteristic of permitting initiation otmotor rotation on very small electromotive forces of the order of 10 microvolts.

'The present invention also relies for its operation' upon the discovery thatapparatus operative to measure the number of lines of induction cut by theexploringcoil is obtained when the motor speedis a linear function of the electromotive force derived from the exploring coil.

. This phenomenon is described in detail herein- The provision of simple and easily adjusted means to eliminate errors due to parasitic'and otherextraneous effects; the provision of simple and eflicient means to adjust the sensitivity of response of the apparatus to the cutting of lines of induction by the exploring coil; and the provision of equally simple and eflicient means to' vary the range of response of the apparatus to lines of induction cut by the exploring coil all constitute additional-objects of the invention.

The present invention relies or its operation I upon the discovery that the speed of rotation of a reversible electrical motorin both directions after in connection with the description of the drawings, but may also be explained mathematically. For example, consider the electromative- F I e=f(t) (1) derived in an exploring coil where e is the instantaneous potential at anytime (1;). Also v J d4, I3: where dqs is the rate of change of flux with respect to time. Hence where 4s is the flux cut in the time (ti-to) Now also consider the relation s-Ke (5) where s is the speed of travel of the exhibiting element or pointer power-set by the reversible motor, and accordingly, is a function oi. the motor speed. since the distance (D) the exhibiting element. travels is a function of the speed of travel and the time of travel, it is seen that represents the distance the exhibiting element travels in time (ti-to). Considering Equations 1, 5 and 6 it is seen that in first one direction and then the other and thentend to be returned to its starting position. In addition, the peak deflection will indicate the maximum number lines of induction cut in any one direction. I

The various features of novelty which chai acterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects obtained from its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described preferred toms of .the apparatus for use in the practice oi the present invention. I

01' the drawings:

Fig. 1- is a diagrammatic representation of one embodiment of the present invention; t

Fig. '2 illustrates in detail the s idewire and switch adjusting mechanism of Fig. 1;

Fig. 3 is a wiring diagram illustrating in simpler manner a portion oi the electronic ampliher and motor drive system of Fig. 1;

Fig. 4 is a partial front view of the apparatus of Fig. 1 showing the arrangement of the various h adiustments;

Fig. 5 is a graph showing the relation between the electromotive force derived in the exploring coil of Fig. 1 and the speed or the reversible motor;

Fig. 6 is a graph showing the manner in which the exhibiting element of Fig. 1 is deflected upon variation in magnitude and polarity of the electromot ve force derived in the exploring coil;

Fig. '7 is a wirin diagram illustrating a modifi cation or the electronic amplifier and motor drive system of Fig. 1; and r Fig. 8 is a wiring diagram illustrating another modification of the electronic amplifier and motor drive system of Fig. 1. In Fig. '1 of the drawings we have illu trated, more or less diagrammatically, an arrangement where e is the generated electromotive force, N.

- tion by means of the present invention to obtain for detecting and visually exhibiting and recording the number of lines of induction in a magnetic field produced by a dipole I. For the purposes of the present invention the nature oi the body constituting the dipole I isimmaterial. Thus, the dipole i may comprise an ordinary bar magnet, a ship at sea which exhibits magnetic properties, or the earth itself.

The presence of the dipole l is. detected by moving a conductor of electricity with respect to it to generate an electromotive force in said conductor which may be measured by suitable means, to be described, to provide an indication of the field strength about the dipole, or in other words the number or lines of induction existing in the path traversed by the conductor. as well as of the presence of the dipole. Specifically, an exploring coil 2 which may comprise about 800 turns of wire is subjected to the magnetic field established by the dipole I, and relative motion between the dipole and coil is effected in any convenient manner to cause the coil 2 to cut the lines of induction about the dipole. It is immaterial whether both the dipole and coil are moved to accomplish such cutting of flux or whether one of said elements is held stationary and the other is moved. Regardless oi how the flux cutting is accomplished an electromotive force is generated in the coil 2 according to the well known formula is the number of turns making up coil 2 and 1 2 dt is the rate at which the lines of induction about the dipole l are cut by the coil 2.

It should be observed that when the rate of travel of the dipole and coil past each other is high the electromotive force generated in coil 2 will also be high, at least in comparison to the electromotive force generated in the coil when the rate of relative travel is low. While .the generated electromotive force is higher in the first case than in the second, it is to be noted that the higher electromotive force will be offrelatively short duration compared to that of the lower electromotive i'orce. Advantage is taken of this relation between the magnitude of the elec-- tromotive force generated in coil 2 and its duraa measure of the total number of lines of induction about the dipole l irrespective of the rate otrelative travel of these components past each other. It is evident that the total number oi.

. lines of induction cut by the coil 2 when moved relatively to the dipole i will be the same regard! less of whether the rate 0! relative travel is low or high. Hence, the indication or record obtained of the total number of lines of induction about the dipole should be the same in each case.

Stated briefly, advantage is taken of the characteristic relation between the magnitude of the generated electromotive force and its duration by providing a reversible electrical motor, designated by the reference numeral 3, to adjustan exhibiting element, designated 4, and by providing an associated motor control system 5 to control the operation 0! the motor 3 in such manner that its speed varies linearly with the magnitude of the generated electromotive i'orce. Consequently, when the generated electromotive Iorce derived in coil 2 is large but of short dura- 8 tion the motor 3 will operate at high speedfor a correspondingly short intervah'and when the tor shaft will be the same in each case, the. total I number of lines of induction cut by coil 2 being the same, and hence, the deflection of the exhibiting element 4 will be the same in each case.

The phenomenon giving rise to an electrometive force in coil 2 as it cuts the lines of induction established about the dipole l is well known and completely through the magnetic field about di-. pole I, an electromotive force is produced in coil 2 which gradually increasesfrom zero. in one-direotion to a maximum value, thereafter gradually. decreases to zero again, builds up in the opposite direction to a maximum value, and then decreases to zero again. The time required forthe hence requires no explanation herein. Suflice it to say that as the coil -2 moves in one direction.

generated electromotive force to complete such, T

cycle of variation thus is dependent upon the speed of travel of dipole fl and coil 2 past each other. In some cases it obviously may be of short induction, for example, of the order of seconds or even fractions of a second, and in other cases it may be of longer duration such as of the order of minutes, hours or days.

The manner in which the electrornotive force generated in coil 2 as a result of its motion relative to dipole l is utilized to so control the speed of operation of motor 3 will now be described.

Referring to Fig l of the drawings, it will be noted that the motor control system 5 includes a vibrator or equivalent device 6, a transformer.

I4. The pulsating potential created bythe'vibrator is converted into an alternating potential I of one phase or of opposite phase depending upon the polarity of the generated electromotive force and is amplified by the transformer I, and is further amplified by the vacuum tubes 9 and 9. The

'output from the vacuumtubes 9 and 91s impressed on the input circuit of the motor drive vacuum tubes l9 and II and operates to control the conductivity of the latter as required to effect selective operation of the reversible motor 3 for rotation in one direction or the other according to the polarity of the electromotive force generated in coil 2 and at a rate of speed directly proportional to the magnitude'of said ,generated electromotive force. The-motor 3 is employed to adjust the exhibiting element 4 relatively to a circular chart 29 which may desirably be calibrated directly in terms of magnetic lines of induetion or fiux. The exhibiting element 4 preferably carries a fountain pen 2| on its free end which engages the chart 29 and makes a permaiii grounding conductor 26.

nent record of the deflections in position imparted to the exhibiting element 4 by the motor 3. If desired, a suitably calibrated indicating scale may be arranged in cooperative relation with the exhibiting element 4 in addition to or in lieu of the chart 29'.

As diagrammatically shown, the primary winding of the transformer I comprises two sections 22 and .23 which have their adjacent ends connected together and to the input terminal i9 of the networksection H, which terminal is connected by conductor I3 to one terminal of the exploring coil 2. The core structure and casing of the transformer 1 and a shield 24 interposed between the transformer primary windings and its secondary winding 25 are connected to a The remote ends or terminals of the primary winding sections 22 and 23 are connected to the stationary contacts 21 and 25, respectively, of the vibrator 9. The latter comprises a vibrating reed 29 carrying a contact moved by the vibration of the reed back and forth between the contacts 21 and 29 which it alternately engages.

The reed 29 is connected by means of the network sections it, I1, i5 and I4 to the other input terminal l9 of the network section i4 which input terminal is connected by the conductor l2 to the other terminal of the exploring coil 2, and is caused to vibrate by a winding 39 having its terminals connected to a source of alternating current. A permanent magnet 3|v is associated with the reed 29 for polarizing and synchronizing purposes; In operation the reed 29 is in continuous vibration with a frequency corresponding to that In'consequence; the pulsating currents flowing alternately through the winding sections '22 and 23 creates an alternating voltage in the secondary winding 25 of the transformer 'l which is in phase with or 180 out of phase with the source of energization for the winding" depending upon the direction of the pulsating currents alternately input terminals l8 and i9 offthe network section flowing through the transformer primary wind- 7, ings 22 and 23, and the magnitude of which corresponds to the magnitude of said pulsating currents.

The alternating voltage so created in the transformer secondary winding 25 is well adapted for amplification by the electronic amplifying and control apparatus which has its input terminals connected to the terminals of th transformer secondary winding 25.

I Said electronic apparatus comprises an amplifying section including the vacuum tubes 3 and 9 andfa motor drive section including the tubes all! and II and motor 3, both of which sections receive energizing current from a'transformer 32 having a line voltage primary winding 33 connected to" the alternating current supply conductors L and L through a double pole-single throw switch S. 1

The motor 3, as diagrammatically shown in Fig.

1 and in more detail in Fig. 2, comprises a rotor 34 having its shaft mechanically rigid with a pinion 35 which meshes with and drives a gear 35.

The gear II also carries a suitably configured cam 40 which rotates in unison with the gear II and operates to impart lateral movement to a resilient contact arm H and thereby adjusts a contact 42 along the length of a short slidewire resistance 43. To this end the resilient contact arm 4I is mounted on the end of a lever 44 which is pivoted at 45 and is provided with a part 4I which bears against the cam surface of the cam 4I; Lever 44 is biased for rotationin a counterclockwise direction by' gravity as seen in Fig. l and also by suitable spring means, not shown.

The short slidewire resistance and associated contact 43 are provided to create a restoring potential to insure that the exhibiting element or pen arm 4 wi1l be returned to a predetermined zero position within its range of movement when no lines of induction are cut by the exploring coil 2. The manner in which this result is accomplished is explained more in detail hereinafter.

The gear II carries a second cam 40A of suitable configuration for operating a switch 41, which may be of the type known as microswitches, whenever the exhibiting element or pen arm 4 is defiected away from. the aforementioned predetermined zero position along the path traversed by it. In order to attain this operation, a resilient arm 48 is rigidly supported at one end by theswitch 4! and is provided with a roller 49 at its other end which bears against the cam surface of the cam 40A. The switch 41 and the mechanism therewith associated constitutes a control means for operating an audible or visual signal, which maybe series connected with switch 41 and the supply mains, or for accomplishing any other control eifect whenever the exhibiting element or pen arm 4 deflects in one direction or the other away froma predetermined position. It will be undermately 90' the voltage of the alternating current supply conductors L and L As will become apparent from the subsequent description, the phase of the current now through the winding l1 and the direction of rotation of the rotor I4 tiepends upon, and is controlled by, the polarity of the electromotive force generated in the coil I. The duration of that rotor rotation depends upon the duration of the electromotive force generated in coil 2. and the speed of rotor rotationdepends upon the magnitude of the generated electrometive force. I

The alternating voltage created in the secondary winding 2! of thetranstormer I is amplified through the action of the amplifying tubes I and I and the amplification thus effected is utilized in energizing the phase winding ll of the motor I to control the selective actuation of the latter for rotation of the rotor I4 in one direction or the other at a speed proportional to the magnitude of said alternating voltage.

As shown, the electronic amplifying tube I includes two heater type triodes enclosed in the same envelope and designated by the reference characters II and II. The triode It includes anode, control electrode, cathode and heater filaidrive tubes III and II have been omitted to simplify'the drawing.

. The electronic amplifying tube 9 includes two heater type triodes, designated by the reference characters II and I2, and enclosed in the same envelope. Both of the triodes of tube 9 include anode, control electrode, cathode and heater filament elements. Each of the motor drive tubes indicate that the number of lines of induction cut form schematically shown in the drawing in which one pair of oppositelydisposed field poles are surrounded by a winding SI connected between the motor terminals 50 and II, and the I other pair of poles are surrounded by a winding 51 connected between the motor terminals II and 54. i

Due 'to the action of condenser 52, the current flowing through the motor winding 58 will be in phase with the voltage of the alternating current supply conductors L and L. The current supplied to the winding 51 will lead or lag by approximately 90 the voltage of the alternating current supply conductors L and L. The windings lit and 51 thus establish fields in the rotor I4 which are displaced from one another by approximately 90 in one direction or the other, depending upon whether the winding 51 is energized III and I l includes two heater type triodes which are enclosed in the same envelope and include anode, control electrode, cathode and heater filament elements. The triodes in'tube II have been designated by the symbols .6! and I4 while the triodes in tube II have been designated by the symbols II and 66.

, The triode 62 of the vacuum tube 8 is utilized as a half wave rectifier providing direct current voltage for energizing the anode or output circuits of the triodes II, 58 and II. As shown, the

tive terminal of filter II is connected by a conductor II to the right end terminal of the transformer secondary winding 61.

The filter 10 includes a condenser I2 which op- I erates to smooth out the ripple in the output voltage of the filter between the points II and 14. The filter 10 also includes a resistance "and a condenser II which operate to smooth out the output voltage of the filter between the points I1 and II. The filter II includes a further resistance I9 and condenser III for smoothing out the output voltage between the filter points II 9 and 62. The filter, therefore, comprises three stages. Such a three-stage filter is provided because for satisfactory and eiiicient operation it' is desirable that the anode voltage supplied to triode 56 be substantially free from ripple whereas it is not necessary to supply anode voltage so completely free from ripple to the output circuit of the triode 56. Likewise, it is not necessary to supply anode voltage as free from ripple to the triode 6| as to the triode 59. a

' The anode circuit of the triode 56 may betraced from the filter point 62, which comprises the positive terminal-of the filter, through a fixed resistance 63 to the anode of the triode 56, to the cathode thereof, and through a cathode biasing resistance 64 which is shunted "by a condenser 65, to the negative filter point 13 through the previously mentioned grounded conductor 26 and a conductor 66. The cathode biasing resistance 34 and the parallel connected condenser 65 are utilized for biasing the control electrode of the triode 56 negatively with respect to the cathode.

The input circuit of the triode 56 may be traced from the cathode to the parallel connected resistance 64 and condenser 65 through the transformer secondary winding 25, and a conductor 61 to the control electrode of the triode 56. As shown, a tuning condenser 25A of suitable value is connected in parallel to the transformer secondary winding 25.

V The output circuit of the triode 56 is resist-- ance capacity coupled to the input-circuit of the triode 59 by means of a condenser 66 and 'two assigns denser 96 and a parallel connected resistance 91 and condenser 96. The condenser 96 is connected by a conductor 69 to the control electrodes of all of the triodes 63, 64, 65 and 66, and is connected to the cathodes of all of said triodes by means of the parallel connected resistance 91 and condenser 96, and a cathode biasing resistance I66. As will be apparent, the signal voltage from the output circuit ofthe triode 6| is impressed simultaneously and equally on all four of the control electrodes of the triodes 63, 64, 65 and 66.

Anode voltage is supplied the output circuits of the triodes 63, 64, 65 and 66 from the high voltage secondary winding 55 of the transformer 32. The anodes of the triodes 63 and 65 are both connected to the left end terminal of the transformer secondary winding 55 and the anodes of the triodes 64 and 66 are both connected to the right the transformer secondary winding 55. Thus, the

triodes 63,64, 65 and 66 are utilized to supply energizing current to the phase winding 51 of the motor 3. It will be noted that this energizing circuit comprises two parallel branches, one including the left end section, of, transformer secondary winding 55 and the, parallel connected anode circuits of triodes 63 and 65, and the series connected resistances 66 and 96. Specifically, the anode of the triode 66 is connected by the condenser 66 to the control electrode of'triode 59 and the control electrode of the triode 59 is connected through the two series connected resistances 69 and 96 to thecathode of the triode 56. The anode circuit of the triode 59 may be traced from the positive terminal 18 of the filter 16 through a fixed resistance 9| to the anode of the triode 59, the cathode thereof, and conductors 26 and 66 to the negative terminal of the filter.

The output circuit of the triode 59 is resistance capacity coupled to the input circuit of the triode 6I by means of acondenser 92 which is con.- nected between the anode of the triode 59 and the control electrode of the triode 6i, and by means of a resistance 93 which is connected between the control electrode of the triode 6i and the oath-- ode thereof. It is-noted the resistances 69 and 96 in the input circuit of triode 59 and the-r-' sistance 93 inthe input circuit of triode 6| operate to maintain the control electrodes of the triodes 59 and 6! at the same potentials as their associated cathodes when no voltage is induced in the transformer secondary winding 25, and upon the inductionof an alternating voltage in the secondary winding 25, the resistances 69, 96

odes 6 3, 64, 65 and 66 by means including a conother including the right'end section of winding 55 and the parallel connected anode circuits of triodes 64 and 66.

Parallel connected triodes 63, and 64, 66 are provided in the motor drive stage to diminish the possibility of failure of the apparatus to function .due to failure of one of the motor drive tubes l6 and ii. It is unlikely that both tubes III and I i will fail at the same time and since one triode of each parallel connection is contained in a different one of the tubes l6 and II, failure of either tube will; not result in failure of the apparatus, as a whole, to function. In addition,

the use of the, parallel connected triodes makes I increased torque from motor} available.

The motor 3 is preferably so constructed that the impedance of the winding 51, together with that of a condenser I63, is of the proper value to match the impedance of the parallel connected anode circuits of the triodes 63, 65 and 64, 66,

energizing current supplied to it. This provides and 9 3 permit the fiow of grid current between for maximum power during the running condi-.

tion of the motor with the least amount of heatin and also provides a low impedance path for braking purposes.

As noted hereinbefore, energizing current is supplied to the motor winding 56 from the alternating current supply conductors L and I? through the condenser 52. The condenser 52 is so selected with respect to the inductance of the motor winding 56 as to provide a series resonant circuithaving a unity power factor. By virtue of the series resonant circuit, the total impedance of the motor winding 56 is substantially equal to the resistance of the winding, and since this resistance is relatively low, a large current flow through the winding 56 is made possible. This .75 permits the attainment of maximum power and torque from the motor 3. In addition, the current flow through the motor winding 66 is In phase with the voltage of the alternating current supply conductors L and LP because of the series resonant circuit. The voltage a cross the motor winding 66, however, leads the current by sub- -stantially 90 because of the inductance of the winding 56.

As will now be apparent, energizing current is supplied the motor winding 51 from the transformer secondary winding 65 through the anode circuits of the triodes 63, 64, 66 and 66. The

condenser I03 is connected in parallel with the motor winding 61 and is so chosen as to provide a parallel resonant circuit having unity power factor. This parallel resonant circuit presents a relatively high external impedance and a rela- I tively low local circuit impedance. The relative- 1y high external impedance is approximately the same as the impedance of the anode circuits of the parallel connected triodes 63, 66 and 64, 66, and hence, provides eilicient operation. The relatively low internal circuit impedance approximates the actual resistance of the winding 51, and since this resistance is relatively low,

' the impedance of the local circuit is also relatively low.

For the first half cycle of the alternating voltage produced across the terminals of the transformer secondary winding 55, the anodes of the triodes 63 and 65 are rendered positive with respect to the center tap I02, and during the second halt cycle, the "anodes of triodes 64 and 66 signal voltage is in phase with or 180 out of phase I with the voltage of the supply conductors 1.. and

Buch energization of the motor winding 61 operates to introduce therein an alternating component of current of the same frequency as that supplied by the alternating current supply conductors L and L. This alternating component or currentwill either lead or lag by 00?. The alternating current flowing through the motor winding 66 depending upon which oi the two pairs 01' triodes 63,60 or 34, 66 have their anode creased with the result that the rotor damping 1 effect is reduced.

In normal operation, the alternating signal voltage impressed on the control electrodes of the triodes 63, 64, 66 and 66 through their connection byconductor 99 and condenser 96 to the anode circuit of the triode 6|, is dependent in magnitude and direction upon the magnitude and direction of the pulsating current flow through the detector circuit including the exploring coil 2, vibrator are made positive with respect to the center tap I02. Accordingly, the pairs of triodes 63, 66 and 64, 66 are arranged'to conduct on alternate half cycles of thealternating current suppliedby'the supply conductors L and L.

When no signal or grid bias is impressed on the control electrodes of the triodes 63, 64, 65 and 66, pulsating unidirectional current of twice the .frequency of the alternating voltage supplied by conductors L and, U is impressed on the core structure of, the motor 3 tends to become saturated whereby the inductive resistance of the motor winding 6! is relatively small. The

6, and the primary winding sections 22 and 23 of the transformer I.

When no electromotive force is generated in the exploring coil 2, no alternating signal voltage is normally transmitted to the control electrodes of the triodes 63,64, 66 and 66 from the anode circuit of triode 6I, and the rotor 34 o! the motor 3 has no tendency to rotate, when this zero input condition is disturbed, due to the generation or an electromotive force of one polarity in the exploring coil 2, the motor 3 willrotate in the direction to move theexhibiting element 4 in one direction. Conversely, when the zero input con- .condenser I03 is shunt to the motor winding 51 and the cathodes biasing resistance I00 are so chosen that the condenser I03, resistance I00 and motor winding 61 then form a parallel resonant circuit. This saturation of the core struc-' ture of the motor 3 operates to exert a damping effect on the rotor 34 of the motor, or in other words, an effect tending to retard rotation of the rotor 34. Consequently, if the rotor has been rotating, saturation of the motor core structure operates to quickly stop the rotation.

When an alternating signal voltage is impressed on the control electrodes of the triodes 63, 64, 65 and 66, the magnitude of the pulses of current dition is disturbed as a result of the generation oi! an electromotiveiorce oi the opposite polarity in the exploring coil 2, the motor 3 will give the exhibiting element 4 an adjustment in the opposite direction. In the operation of the apparatus, the rotation of the motor 3 is substantially contemporaneous with the detector current flow which is indicative of the need for the motor rotation.

As'will be apparent by reference to Fig. 3, the

phase winding 61 of the reversible electrical motor comprises one arm of a bridge network I04. An adjacent arm of the bridge network I04 is comprised of the cathode biasing resistance I00. A third bridge arm includes a condenser I05 and a resistance I06 connected in series while the fourth bridge arm includes a resistance I01. The

input terminals of the bridge network I04 have triodes 63, 65. or 64, 66 will be increased while the magnitude of the pulses of current flowing in the anode circuits of the other pair of triodes will be decreased. Accordingly, the pulses of unidirectional current supplied to the motor winding 61 during the first half cycle will predominate over those supplicd the motorwinding during the second half cycle.v WhLle current pulses'will be increased de ends upon whether the alternating been designated by the reference numerals I00 and I00 which are respectively connected ,to the cathodes of the triodes 63, 64, 65 and 66 and to the center tap I02 on the transformer secondary winding 65. One output terminal IIO oi the opposed arms.

condenser I05.

the speed of rotation of the motor rotor 34 to vary ina linear manner with the magnitude of the electromotive force generated in theexploring coil 2 over a much wider range of variation I of that electromotive force than would beotherwise possible if the bridge network I04 were not provided. For example, we havedetermined experimentally than when the, bridge network I04 is provided, the rotation of the motor 3 may be gradually varied from a standstill condition up to full speed as the electromotive force impressed on the detector circuit including vibrator and the primary winding sections 22 and 23 of the transformer 1 is varied from substantially zero up to approximately 1700 microvolts, whereas the motor will be varied from a standstill condition to full speed as the said electromotiveforce is varied from substantially zero to 30 microvolts or less when the bridge. network I04 is not provided. This wide range of linear response is obtained, moreover, without any significant sacriflce in regard to the input electromotive force re- ,quired to initiate motor operation, motor rotation being initiated by an electromotive force of 10 microvolts or less.

This result is obtained by deriving from the bridge network I04 a voltage of the same frequency as the voltage of the supply mains L and L, the magnitude of which is a linear function of the motor speed and the phase of which is determined by the direction of rotation thereof, and by introducin this voltage into the input circuit of triode 59 in opposition to the alternating signal voltage impressed on that input circuit from the output circuit of triode 50.

The bridge network I04 is of a type known in the art as' a Hay bridge and includes the fixed resistances I00 and I01 in twodiametrically opposed arms and capacitance and inductance,

respectively, in each of the other diametrically The inductance is that of the motor winding 51 and the capacitance is that of As previously noted, the bridge arm including condenser I05 includes the fixed resistance I06. 7 The value of condenser I05 is so chosen as to ofiset, at least to a substantial extent,

and if desired, entirely, the effect of the inductance of motor winding 51 so that when the motor rotor 34 is at rest the bridge network I04 is substantially balanced, and hence, little or no unbalanced voltage, useful for extending the range of response of the motor control system 5 to the generated electromotive force of coil 2, appears at the bridge output terminals H0 and III.

On rotation of the motor rotor 34, an electromotive force of, one phase or of opposite phase relatively to the .voltage of the supply mains L and L and of the same frequency is produced at the bridge output terminals I I0 and II I. The magnitude of this electromotive force is dependent upon and varies in a linear manner with the speed-o1 rotation of the rotor 34, and is impressed through conductors 20 and H2 across a portion of the resistance 00 depending upon the adjustment of contact I I3. The said electromotive force' is so fed back to the resistance 90 as to oppose the fluctuating electromotive force produced across the latter as a result of the production of an electromotive force in the exploring coil 2. The amount of the said electromotive force which is fed back may be adjusted by manipulation of contact I I3 along resistance 00.

' The precise manner in which an electromotive forceis produced at the bridge output terminals I I0and I I I upon motor rotation'is not now known to us, but it is believed to be a complex quantity created by two eflects, one of which is due to transformer action between the motor windings 5i and 5,1 and the other of which is due to change in impedance of the winding 51. These two eil'ects are additive, since the electromotive force created at the bridge output terminals H0 and III by one of the eflfects augments the electromotive force there produced by the other efiect.

The transformer action effect will be first explained. Since the motor windings 56 and 51 are displaced 90 with respect to each other on the core structure of the motor, no lines of magnetic flux established by the winding 56 link any turns of the winding 51 when the rotor 34 is stationary, and hence, no electromotive force is then induced in winding 51 as a result of transformer action between windings 50 and 51. Upon rotation of therotor 34, however, the magnetic flux established by the winding 50 is distorted whereupon some lines of said magnetic flux link the turns of winding 51 to induce an electromotive force in the latter of the same frequency as the voltage supplied by mains L and L The induction ofv this electromotive force in winding 51 causes an electromotive force of the same frequency to appearat the bridge output terminals H0 and III. The magnitude and the phase of this electromotive force relatively to the voltage of the supply mains L' and L varies in accordance with the speed of rotation of the rotor 34 and the direction of rotation thereof,.respectively, inasmuch as the extent of distortion of the magnetic field of winding 56 is dependent upon the speed of rotation of rotor 34 andthe direction of distortion is determined by the direction of rotor rotation.

The manner in which a component of electromotive force is produced at the bridge output terminals I I0 and III upon motor rotat'on due to change in impedance of the winding 51 brought about by such rotation will now be explained. As noted hereinbefore, the constants of the bridge network I04 are so chosen that at the frequency of the voltage supplied by supply conductors L and L, the bridge network I04 will beexactly balanced. The value of condenser I05 isv then eflective to cancel out the inductive effect of motor wind'ng 51. Upon motor rotation, however, the effective coupling between the motor windings 56 and 51 is changed and this effective change in coupling produces an apparent change in inductance and also in resistance of winding 51.. At such new apparent value of inductance and resistance of winding 51 the condenser I05 is not operative to balance the bridge network to the same extent as when the motor is at rest, and

hence, the bridge network I04 becomes unbalaccordance with the speed of rotation of the rotor 34- since the apparent chan e in impedance of winding 51 is dependent upon the speed of rotation ofrotor 34, The change in inductance of winding 51 with motor rotation is always in the same direction regardless of the direction of rotor rotation, and hence, the bridge network is unbalanced in the same directionirrespective of the direction of motor rotation.

Although the bridge network I04'is always unbalanced in the same direction regardless ofthe direction of motor rotation,-such unbalancefis nevertheless operative to cause'the production of an electromotive force in phase with the voltage of the supply lines L and L to appear at the bridge output terminals H and I ll upon rotation oi the motor in one direction. and to cause the production of an electromotive force of the opposite phase at the bridge output terminals upon rotation of the motor in the opposite direction. Such phase shift upon reversal in the direction of motor rotation is produced because pulsating voltage in phase with the voltage of the supply mains L and IF is impressed on the bridge input terminals llll and I" when an electromotive force of one polarity is impressed on the detector circuit including vibrator and the primary winding sections 22 and 23 of the transformer I. and pulsating voltage of the opposite phase is impressed on the bridge input terminals when an electromotive force of the opposite polarity is impressed on that detector circuit. In the first case the motor 3 is energized to rotation in one direction, and in the second case the motor is energized to rotation in the opposite direction. Consequently, the same change in impedance of motor winding 51 produces an electromotive force of one phase at the bridge output terminals ill! and II i when the motor is enersized for rotation in one directionand produces an electromotive force of opposite phase at the bridge output terminals when themotor is energized for rotation in the opposite direction.

As those skilled in the art will recognize, the bridge. network IN is of a type which may be exactly balanced only at one frequency of the energizing electromotive force impressed on its input terminals. In practice, the bridge components are so chosen that the bridge network is balanced when fluctuating voltage of the frequency of the supply mains L and L is impressed on its input terminals I" and I05, In the arrangement 01' Figs. 1 and 2 the voltageimpressed on the bridge input terminals I08 and It! also includes a component of twice the frequency oi the voltage supplied by conductors L and 1?, and accordingly, an electromotive force oi this higher frequency will be produced at the ,bridge output terminals H0 and ill and will be impressed through the feedback conductors 2i and H2 across the resistance 90 in the input circuit of triode 59 even when the rotor 34 of motor I is at rest. This higher frequency component is not effective to actuate motor 3 for rotation, however, since the motor drive stage including triodes 53, 64, 55 and 68 and the transformer secondary winding 55 is of a frequency discriminating type and, will not respond to voltages of twice the irequency of that supplied by the supply conductors L and L for energizin motor 3 to rotation.

As may be seen by reference to Fig. 4 which shows a partially broken away front view ot a practical embodiment of Our invention, the api6 nated by the reference character A and driven by the reversible motor 3 in unison with the exhibiting element or pen arm 4 so as to make substantially one complete revolution as the element 4 moves over its range 0! movement may also be provided as shown.

On the upper panel ill just inside the splashproot door I there are four control dials and a switch. The first control dial on the left. as seen in Fig. 4, has been designated by the reference symbol "hand is marked Restoring Voltage. This control dial controls the energizing voltage impressed on the short slidewire resistance 43 01 the electrical network section II from a battery HI, as may be seen in Fig. 1. Battery In, which may be an ordinary dry cell, has its positive terminal connected through a resistance ill to one terminal 0! a switch HI. The other terminal oi switch Ill is connected to a contact "B which is in slideable engagement with a slidewire resistance "C. The upp r terminal of slidewire resistance C, as seen in the drawing, is connected to the negative terminal of battery H1 and also to the resilient arm 4| which carries the slidewire contact 42 on the end thereof. The lower end of resistance HC is connected to the adjacent terminals of a pair of equal resistances I20 and HI which are connected in series between the terminals of the slidewire resistance 43. A resistance I22, connected in parallel to the slidewire resistance 43 and also the series connected resistances I20 and III, is also provided in the electrical network section II.

The control dial HA is provided toiacilitate ad- Justment of contact I'IB along the length of slidewire resistance HC and thereby to facilitate ad- Justment oi the potential drop impressed across the slidewire resistance "C from thebattery 1.

It will be noted that the slidewire resistance 43 and these'ries connected resistances m and I'M comprise a bridge network III the input terminals of which comprise the contact 01 in engagement with the slidewire resistance l3 and the point of connection 01 the adjacent ends of resistances I20 and III. The output terminals .comprlse the remote ends I and I25 oi resistances I10 and I2! and are shunted by the resistance 412. When the contact 42 is in a central position along the length of the slidewire resistonce '43, -a position corresponding to the zero position of the exhibiting element or pen arm 4, the bridge III is balanced and the potentials of the bridge output terminals I24 and I25 are identical.

Upon rotation of the motor 3 in one direction and consequent deflection oi the contact 42 in a paratus shown in diagrammatic manner in Figs.

1' and 3 may be so constructed that certain of its functions can be easily varied as desired, by an attendant. The instrument shown in Fig. 4 com prises a rectangular casing Ill which contains all of the various apparatus components except the exploring coil 2. Ordinarily the exploring. coil 2 is located at a position remote from the remainder of the apparatus. The casing H4 is provided with a door H5 which is hinged in any convenient manner to the casing Ill and is provided with suitable gaskets and arranged as required to keep the interior of the casing dry even when the instrument is subjected to spray for extended periods. an indicating pointer desiscorresponding direction along slidewire resistance 43, however, a unidirectional electromotive Iorce .of one polarity is produced between the bridge output terminals I24 and I25. This unidirec-- tional electromotive force is of a polarity tendll'lg to eiiectuate rotation of the motor 3 in the proper direction to restore the exhibiting ele-- ment or pen arm, to its zero position. Consequently; ii the electromotive force which gave rise initially to the motor operation, namely, that generated in exploring coil 2, should decrease to zero in such manner that the exhibiting element or pen arm 4 is left in a deflected position, the

restoring voltage derivedfrom the bridge network lfl will operate to restore the saidexhibitlng member 4 to its zero position.

It will be evident that upon rotation of the motor I in the opposite direction and resulting deflection oi the contact I! in the opposite di- I along the chart 20 regardless of the existence of such stray electromotive forces. As may be seen "by relerence to Fig.1, the control dial, ISA controls to zero, while the said exhibiting member i s in a deflected position. I I F Accordingly, the network-section II including the short slidewire resistance 43 land associated contact 42 driven by motor .3 ensures theproper return of the exhibiting member 4 to 1125115110130! sit-ion along the chart 'I.- In this connection it U will be noted that by employing a cam 40 of props er configuration for imparting lateral motion to the. contact 42 relatively to the slidewire resistance '43,the contact 42 maybe moved over the entire length of slidewire resistance 43 upon only slight departure of the exhibiting element or pen arm 4 from its zero position,z: for example, upon departure of approximately ten percent of the full scale travel of the member 4. To this end the slidewire resistance 43 is preferably provided with relatively long terminals 43a so as to permit slight motion of the contact 42 relatively to the slidewire resistance 43, when the contact is in its extreme deflected positions, without causing any 5 change in the restoring voltage; 1

As those skilled in the art will recognize, the restorin voltage so obtainedand utilized also provides a simple and eflicient method of attenuating electromotive force variations of longer pe- 5 riod which may be produced in theexploring coil 2 due to the presence oflextranecus magnetic fields not underobservation or otherwise impressed on the instrument detector circuit."

The control dial IlA.-as seen in'Fig. 4, is cali-v brated from 0 to 10 and each division represents 3 l0 mi'crovolts. Hence, it the knob isset on 5, there will be made available 50 microvolts, when the contacts 42 is deflected toeither extreme position, for suppressing'the electromotive force impressed on terminals I3 and I3 from exploring coil 2 to attenuate undesired long period signals and 'to bring the exhibiting member 4 back to its zero position in the event the signal under observation rails. Inasmuch as the use of large restoring voltages would not "allow the motor- 3 i to drive'beyond the range of the slidewire resistance 43 on small signals and would also attenuate signals of medium period, mr examp1e,'of-' the order of 15 seconds, the restoring voltage is pref erablyalways smaller'thandoll microvolts; As

a result, the rate atwhich the exhibiting member-4, is restored to its zero position upon failure of the observed signal is generally quite slow. The actual amount of restoring voltage required may be determined experimentallyby the operator and adjusted by him to that value required to'attenuate the contemporaneous long period signals which are undesired.

The'second control dial from is a vernier rheos't'at which has been designated bythe reference character IBA'and is marked Centering. Its function is to provide a com-- pensating ele'ctromotiveiorce to compensate for theenergizing voltage impressed onthe detector circuit including elements 5, 22 and 23 from a battery I26 which may comprise anordinary dry cell similar to the battery I" and'is connected in the electrical network section I6. The dual vernier rheostat is comprised-of two resistances bridge I23, while the other terminal of resist-' ance I29 is connected to the vibrating element 29 of the vibrator 6. Contact I29 associated with resistance I21 is connected? to the positive terminal of battery I26 the negative terminal of which is connected to one terminal of each of a pair of flxed resistances I32 and I33. The other terminal of resistance I32 is connected to the c0ntactI3Il' associated with resistance I28. The other terminal of resistance I33 is connected to the conductor I3I which connectsthe bridge output terminal I24 to one terminal of resistance The control dial ISA preferably has a. direct mechanical connection with the contact I30 and has a lost motionconnection with the contact I29 so that uponinitial movement of the-dial the contact I30 is first moved relatively to resistance I28 through a range in which the contact I29 is stationary with respect to resistance I21 and then contact I29 is moved relatively to resistance I21, thereby providing a vernier or line adjustment and a coarse adjustment. To this end, the resistance I2'I is-preferably of 'higherresistance than the resistance I28. Preferably the circuit components I25 through I33 are so chosen in relation to theremainder of the apparatus that the controldial IBA has'suflicient latitude to compensate for or balance out at least 6 millivolts of unidirectional stray r electromotive force of either polarity.

The third control from the left in Fig. 4 is'the switch II8, mentioned heretofore. The switch I I3 is connected in the electrical network section I! ingsuch manner that when depressed it eliminates the restoring voltage from the detector circuit. Switch H8" is provided so that the restorvoltage were not eliminated during-this compenthe left in Fig. 4

flxed' stray unidirectional electromotive forces 7 which may be introduced into the detector circuit including the vibrator 6 and the primary windings 22 and23 of the transformer I, and in this manner to ensure that the exhibiting element or pen arm 4 will be adjusted to zero position sating adjustment, it would tend to adversely affect that adjustment and might veveniniiuence the adjustment to such an extent as to makeit impossible of accomplishment; It should be noted that the provision of switch H3 makes it possible to eliminate the restoring voltage from the detector circuit while the compensating adjustment is being made without changing the setting of the control dial "A. I

The next control dial from the left in'Fig. 4, designated by the reference symbol HA. is employed as an attenuator for controlling the magnitude of the electromotive forcewhichis impressed on the detector circuit from the exploring coil 2. This control dial is marked Sensitivity" and is included in the electrical network 19 section I4 of Fig. 1. The electrical network section I4, as seen in Fig. 1, also includes a slidewire resistance I34 the terminals of which are connected between the instrument input terminals I3 and I9, and also includes a fixed resistance I35. One terminal of resistance I35 is connected to the input terminal I3, and hence. to onev t rminal of resistance I34, and the other terminal of resistance I35 is connected to a contact I36 which is disposed in slideable engagement with the resistance I34. The last mentioned terminal of resistance I35 and the terminal of resistance I34 which is connected to'the input terminal I3 constitute the output terminals of the network section I4.

The electrical network section constitutes amodified "L-Pad" attenuator inasmuch as the values of resistances I34 and I35 are so chosen that the resistance between the output terminals remains substantially constant regardless of the position to which the contact I36 is adjusted along the length of resistance I34. In this manner any desired fraction of the electromotive force impressed on the terminals I8 and I3 may be impressed on the detector circuit including the vibrator 6 and the primary winding sections 22 and 23 of the transformer 1 without causing any change in the resistance of the detector circuit "looking into" the detector circuit from the terminals I8 and 13. 'It will be noted that the provision of such means for varying the sensitivity of response of the apparatus to the electromotive force impressed on terminals I3 and is from the exploring coil 2 does not alter to any significant extent the effect of the restoring and centering voltages upon the operation of the apparatus.

The electrical network section I5 which has been previously referred to comprises a filter made up of suitable resistive and capacitive components designated by the reference characters lie and IN), respectively, and is inserted between the output terminals of the electrical network section I4 and the detector circuit for the purpose oi filtering out relatively high frequency components of current which may be extraneously induced in the exploring coil 2 or the connecting leads I2 and I3 and which, if not filtered out, would otherwise adversely affect the operation of the apparatus.

The remainingcontrol dial shown in Fig. 4 has been designated by the reference symbol 30A and is marked "Feedback." This control dial is calibrated from to 10 and provides a control to regulate the amount of the output voltage produced by the bridge network I04 which is fed back in opposition to the alternating signal voltage appearing across the resistance 30 connected in the input circuit of the triode 53. a

The curves of Fig. 5 show the manner in which the range of response of the apparatus may be varied by adjustment of the setting of the feedback control dial 90A. All of these curves were obtained by employing a reversible motor 3 of medium speed which would actuate the exhibiting member 4 over its full range of travel in approximately 11 seconds, and with the restoring voltage control dial IIA set at its zero position.

Referring to the curve A of'Fig. 5 it will be noted that with the feedback control dial set at zero so that there is no feedback electromotive force impressed on the input circuit of triode 53 from the output circuit of bridge network I04, the motor 3 reaches full speed with approximately 70 microvolts impressed on the apparatus input terminals I 8 and I3. It has been ascertained that if the filter network II of Fig. l is eliminated that the motor 3 will reach full speed with 30 microvolts or less impressed on the input terminals Iland I3. As the feedback electromotive force is gradually increased by manipulation of the controldial A, the value of the electromoof approximately 250 microvolts while full motor speed is obtained with anunput electromotive force of approximately 11 0 microvolts when the said control dial is set at as is represented by the curves 0 and D,- respectively. 1

Curves A, B, C and'D of Fig. 5 show that thespeed of motor 3 varies in a substantially linear manner with the electromotive force impressed be actuated to rotation when an electromotive force of the order of 10 microvolts or less is impressed on the apparatus input terminals II and I3.

Fig. 6 is a graph illustrating the manner in which the exhibiting member 4 is deflected when the exploring coil 2 is moved completely through the magnetic field established by the magnetic member I. In Fig. 6 the curve E represents the electromotive force generated in the coil 2 as it is moved into the ,magnetic field about member I,

and curve F represents the generated electromotive force as the motion of coil 2 is continued in the same direction to move the coil 2 out of the region of said magnetic field. When the coil 2 is initially moved into the magnetic field, the polarity 01' the magnetic field is such as to cause the induction of electromotive force E of one polarity in coil 2. As the motion of coil 2 continues it eventually reaches a position wherein all of the lines of induction of the magnetic field are parallel to the plane of the turns of coil 2 and in this position no electromotive force is induced in coil 2. This position is indicated by the point G in Fig. 6. Upon further forward motion of coil 2 it will be noted that the polarity of the lines of induction of the magnetic field will be reversed with respect to the coil 2, and consequently, an electromotive force of opposite polarity, indicated by curve F, will be" induced in coil 2. Continued movement oi coil 2 relatively to the magnetic field will eventually bring the coil 2 to a position in which no lines of induction are cut by the coil and in this position no electromotive is induced in the coil.

When the electromotive force of one polarity represented by curve E is impressed on the instrument input terminals I3 and I9, the reversible motor 3 will be energized for rotation in one directlonto saw. th exhibiting member ,4 away from its zero position inthe direction asindicated by thecurve H-of Fig. 6. "The extent to which the exhibiting me'mber4 is'so adjusted is a functionof' both themagnitude ofthe generated; electromotive force E in coil 2 and also the duration of that electromotiveforce. It will be observed that thelspeed of adjustment of member force E decreases to zero as at G, the exhibiting member 4 will come to rest at the deflected position indicated by the reference symbol I. r

Upon reversal of the electromotive force generated in coil 2, as indicated by curve F, the reversible motor 3' will be energized for rotation in the opposite direction and actuate the exhibiting member 4 in the reverse direction to return it to its original zero position. The extent to which the member 4 is so adjusted is a function of both the magnitude of the generated electromotive force F in coil 2 and also the duration of that electromotive force.

If the factors giving rise to the electromotive force represented by curve F are identical to those creating the electromotive, force represent-' ed by curve E, the exhibiting member 4 will be re-' turned to its original zero position and come to rest as the electromotive force F decreases to zero. If those factors are not the same, the exhibiting member 4 will be at a position other than its zero position when the electromotive force F decreases to zero. but will nevertheless be slowly returned to its original zero position due to the action of the restoring electromotive force previously described and created by the electrical network section II. v

Merely by way of. illustration, it is noted that when the voltage of each of the batteries I I1 and I is 1 /2 volts, correspondingly suitable values for thevarious components of the electrical network sections I4, I5, I5 and Il may be as follows:

' Also, when the tubes I0 and II are ofthe commercially available 7N7 type, the voltage produced between the center tap I02 and each end of the secondary winding 55 of transformer 32 is 275 volts, and the inductance of the motor phase winding 51 is 10 henries, correspondingly suitable Part Value H3. ohms 15,000 I'IC do 1, 000 43--- do 200 I20 do 1,000 I2I; do 1,000, I22 do 3' I21 do 100 m do 10 I32 do- 25,000 I33 do 25,000 I34 do 5,000 I35 do 509.5 I5a do 400 I5b mfd 500 values of the various components of the feedback bridge network I04 may be as follows: Part Value I00 "ohms; 150 I05 mi?d 0.1 I06 ohms 15,000 I01 do 535,000 In Fig. 'l we have illustrated, more or less diagrammatically, a modification of the feedback controlmeans of Figs. land; 3 for causing the speed-of rotation of the motor rotor 34 to vary in a linear manner with themagnitude of the electromotive force generated in the exploring coil 2 over a wide range of variation of that electromotive force. With this modificationit has been found that the linear relationship between thle motor speed and the magnitude of the generated electromotive force of coil 2 is somewhat better than'that whichis' obtained by means of the arrangement of Figs. l. and3'. This modified arrangement also is capable of so regulating the energization of motor 3 for rotation that the motor speed may be gradually varied from zero speed up to full speed as the electromotive force impressed on the apparatus input terminals I 0 v and I3 is varied from zero to 1700 microvolts. Such range expansion, moreover, is accomplished while retaining the desirable operation of initia tion of motor rotation on very small electromotive forces, for example, of the order of 10 microvolts or less.

As shown, the feedback control means of this modification includes a. generator which has been generally designated by the reference numeral I31 and is provided in place of the electrical bridge network I04 of Figs. 1 and 3. The generator I3'I may be termed afield distorting generator and its construction has been shown as being identical to that of the reversible motor 3. It is shown as having two field windings I33 and I3! which are wrapped around the stator pole pieces in such manner as to be displacedfrom each other by The field winding I33 is connected to the alternating current supply lines L' and L throu'gha condenser I40 of suitable value. The rotor I may be provided with rotor bars similarly to the rotor 34 of motor 3 or,'if desired, may comprise merely a thin copper cylinder on an iron core. The shaft of'rotor I is mechanically coupled to the shaft of motor 3 so that the two rotors HI and 34 rotate in unison.

When the rotor I is stationary, no electromo- 45 tive force is induced in the field winding I33 from L the winding I38, but as the rotor I is rotated the lines of induction established in the rotor bythe field winding I38 are distorted whereby some of the said lines of induction are caused to pass through the field winding I33. The amount of distortion gradually increases fromzero at zero speed of rotation of the rotor to substantially90 at high speeds.

The electromotive force induced in field winding I33 is impressed on a pair of series connected resistances HI and I42 by means of conductors I43 and I44. One end of the resistance I42 is connected directly to the grounding conductor 28 and a contact I45 in slideable engagement with the resistance I is connected by a conductor I40 in which a resistance I41 is inserted to the cathode of triode 53. A condenser I43 of suitable value is connected between the cathode of triode 53 and the grounding conductor 26. Thus, the feedback electromotive force is arranged to be impressed on the input circuit of the triode 53 in this modification instead of on the input circuit of triode 53 as in Figs. 1 and 3.

/ It will be noted that the resistances I4 I, I42 and I" and the condenser I43 are employed in the modification of Fig. 7 in place of the parallel connected resistance 34 and condenser 35 of the arrangements of Figs. 1 and 3. Furthermorefthe resistance elements 33 and 90 in the input circuit of the triode 53 in Figs. 1 and 3 have been replaced by a fixed resistance I", and the bridge network I in the cathode circuits of the triodes B3, 84, 55 and 88 has been replaced by a single cathode biasing resistance I50. In addition, the condenser I connected in shunt to the resistance 91 in the input. circuits of the triodes 83,64, 65 and 56 has been eliminated. Otherwise the electronic amplifying section and the motor drive section of the modification of Fig. 7 is the same as that of the arrangement of Figs. 1 and 3.

The value of condenser I40 is so chosen in relation to the inductance of the generator field windings Ill and I39 that upon motor rotation an electromotive force substantially in phase with the voltage of the supply lines L and L' or 180 out of phase therewith, depending upon the direction of motor rotation, will be induced in the generator field winding I38. This electromotive force is impressed on the input circuit of the triode 58 in such manner as to oppose the alternating signal voltage impressed thereon from the transformer secondary winding 25. With this modification, as well as the apparatus embodiment of Figs. 1 and 3, therefore, the rotation of the motor 3 may be regulated by the feedback electromotive force in such manner that the motor will have a definite speed for each different value of the electromotive force impresed. on the input terminals I8 and I9 from the exploring coil 2 over a wide range of variation of that input electromotive force. The amount of the feedback electromotive force, and hence, the range of variation of the input electromotive force over which the motor speed may be varied from zero to full speed may be adjusted, as desired, by movement of the contact I45 along the length of the slidewire resistance I. To this end the contact I" may be adjusted along the length of resistance Ill by manipulation of a control dial IIIA'.

In Fig. 8 we have illustrated, more or less diagrammatically, a modification of the apparatus of Figs. 1 and 3 wherein a selenium type rectifier arrangement generally designated by the reference symbol I5I is employed in lieu of the feedback bridge network IIII for regulating the energization of the motor 3 for rotation as required to cause the speed of motor rotation to vary in a linear manner with the generated electromotive force of exploring coil 2 over a wide range of variation of that electromotive force. It will be apparent, upon examination of Fig. 8, that the use of this modification permits a substantial reduction in the amount of equipment involved.

The selenium rectifier feedback network I5I is connected between the cathode circuit of the triode 5i and the cathode circuit of the triode 5! and is utilized as a resistance which varies automatically to change the amount of feedback electromotive force from the output circuit of triode BI to the input circuit of triode 59 in accordance with the magnitude of the alternating signal voltage produced in the output circuit of triode 5|. Consequently, the electromotive force so fed back will be small when the said alternating signal voltage is small whereby motor initiation on small alternating signal voltages is made possible.

As shown, a cathode biasing resistance I52 is provided in the input circuit of triode 59 and a cathode biasing resistance I53 is provided in the input circuit of the triode 6|. One terminal of the selenium rectifier feedback network I5 I comprising the point of connection of one end of each of a pair of selenium rectifier elements which are oppositely connected in parallel, is connected by a conductor I56 to the cathode of triode 59, and the other terminal thereof is connected by a conductor I51 to the cathode of triode 8|. In addition to the rectifier elements I54 and I55 the selenium rectifier feedback network includes a condenser I55 of suitable value which is connected in the conductor I58. While the rectifier elements I54 and I55 have been described as being selenium rectifier elements, it will be understood that these rectifier elements may be of any other type such as copper-oxide rectifier elements. Furthermore, rectifier elements need not be employed inasmuch as any type of resistance element, the resistance value of which is a function of the electrical current flow which the said rectifier elements conduct may be utilized. This characteristic operation of selenium rectifier elements is utilized in the modification of Fig. 8 in order to regulate the amount of feedback electromotive force from the output circuit of triode 5I to the input circuit of triode II in accordance with the magnitude of the alternating signal voltage.

Selenium rectifier elements are characterized in that their resistance values are relatively high in the conducting direction when the current passed is small, but become very low as the conducted current increases. In order to permit current conduction in both directions between the output circuits of triodes 59 and ti, two such rectifiers connected oppositely in parallel are provided.

In a practical, working embodiment 'of this modification of our invention, the rectifier elements I54 and I55 were so chosen that the resistance of the dual rectifier unit was approximately 300 ohms which made the percentage of feedback electromotive force low on small alternating signal voltages, and consequently, allowed the motor 3 to be energized sufficiently to rotate even though the alternating signal voltages were small. As the current flow through the dual rectifier unit gradually increases, however, its resistance gradually decreases to a few ohms, thereby gradually increasing the feedback electromotive force.

Subject matter disclosed in this application but not claimed herein is disclosed and claimed in the application for patent of Walter P. Wills. filed December 1, 1941, Serial No. 421,173, now Patent No. 2,423,540 of July 8, 1947.

While in accordance with the provisions of the statutes, we have illustrated and described the best form of embodiment of our invention now known to us. it will be apparent to those skilled in the art that changes may be made in the form V of the apparatus disclosed without departing from th spirit of our invention as set forth in the appended claims, and that in some cases certain features of our invention may be used to advantage without without a corresponding use of other features.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is as follows:

1. The method of obtaining a measure of the strength of a magnetic field which consists of the steps of relativel moving a magnetic field and an exploring device to derive an electrical effect the direction of which corresponds to the direction of said magnetic field and the magnitude of which is a function of the strength of said magnetic field and also of the rate of relative movement of said magnetic field and said exploring device. applying said electrical efiect to motor means to control the energizati'on of said motor means ior operation of the latter in a direction corresponding to that oi said electrical eflect for a period the duration of which corresponds to that of said electrical effect and at a speedwhich is in substantially linear accordance with the magnitude of: said electrical efiect," utilizing the operation of said motor means for moving an'exhibtting' member from a predetermined position in a direction corresponding to the directionof operationoi said motor means for a period the duration of which corresponds to that of the operation of said motor means and at a rate which czrresponds' to the speed of operation or said motor means, utilizing the operation of said motor means for deriving a second electrical eflect the magnitude of which is a function of the speed of operation of'said motor means, opposing said electrical effect in the control of the energizationoi said motor means to suppress the increase in the rate of motion of said exhibiting member tending to be efi'ected by operation of said motor means as said first mentioned electrical eflect increases in magnitude, thereby to expand the range ot'variation oisaid first mentioned electrical eiIect over which the rate of movement of said exhibiting member varies from zero to a maximum, and additionally controlling the energization of said motor means in accordance with the position of said exhibiting member as required to restore and maintain said exhibiting member in said predetermined: position when the magnitude of said first mentioned electrical effect decreases below a predetermined value.

2. The method of obtaining a measureoi the strength of a magnetic field which consists of the steps of relatively moving a magnetic field and an exploring device to derive an electrical effect the direction of which corresponds to the direction of said magnetic field and the magnitude of which'is a function of the strength of said magnetic field and also of the rate of relative movement of said magnetic field and said exploring device, applying said electrical\ effect to motor means to control the energization of said motor means for operation of the latter in-a direction corresponding to that of said electrical effect for a period the duration of which corresponds to that of said electrical efiect and at a speed which is in substantially linear accordance with the magnitude of said electrical effect, utilizing the operation of said motor means for moving an exhibiting member from a predetermined position ma direction corresponding to the direction of operation of said motor means for a period the duration of which corresponds to that of the ope'rationof said motor means and at a rate which corresponds to the speed of operation of said motor means, continuing such control of the energizationof said motor means while the magnitude of said electrical eifect is greater than a predetermined value, and additionally controlling the energization of said motor means in accordance with the position'of said exhibiting memher as required to restore and maintain said'exhibiting member'in said predetermined position when the magnitude'oi sald'electrical effect decreasestbelow a predetermined value.

3. The method of obtaining a measure of the strength of a magnetic field which consists of the, steps of relatively moving a magnetic field and an exploring device to derive an electromotive force the magnitude of which is a function of the concentration of the lines of induction in said magsecond electrical effect and the first mentioned netic field and also oi! the rate of relative movement 10f said magnetic field and said exploring device and the polarity or which is determined by the 1 direction of said lines of induction with respect to the direction'ot relativemovement of said magnetic field and said exploring 'devi'ce, translating said electromotive force into an electromotive force having an alternating component oi magnitude and of one phase or of -.opposite phase determined respectively bythe magnitude and polarity of the first mentioned electromotive iorce, amplifying said alternating component, applying said amplified component to motor means to control the energization of said motor means for operation of the latter in a direction corresponding to the phase of said amplified component for a period the duration of which corresponds to that of said amplified component and at a speed which is in substantially linear accordance with the magnitude of said amplified component, utilizing the. operation of said motor means for moving an exhibitingmember from a predetermined position in a direction corresponding to the direction of operation of said motor means for a period the duration of which corresponds to that of the operation of said motor means and at a rate which corresponds to the speed of operation of said motor means, utilizing the operation of said motor means for deriving a second alternating component of electromotive 7 force the magnitude of which is a function of the speed of operation of said motor means and the phase of which corresponds to the direction of operation of said motor means, and opposing said second component and said amplified component'in the control of the energization of said motor means to suppress the increase in the rate of motion of said exhibiting member tending to be effected by said motor means as said amplified component increases in magnitude, thereby to expand the range'of'variation of said first mentioned electromotiv force over which the rate of movement of said exhibiting member varies from zero to a maximum.

4. The method of obtaining a measure of the strength of a magnetic field which consists of the steps of relatively moving a magnetic field and an exploring device to derive an electromotive force the magnitude of which is a function of the concentration of the lines of induction in said magnetic field and also of the rate of relative movement of said magnetic field and said exploring device and the polarity of which is determined by the direction of ,said lines of induction with respect to the direction of relative movement of said magnetic field and'said exploring .device, translating said electromotive force into plified component for a period the duration of which corresponds to that of said amplified component-and at a speed which is in substantially linear-accordance'with the magnitude of said amplified component, utilizing the operation of said motor means forqmoving an exhibiting member from a predeterminedposition in a direction corresponding to the direction of operation of said motor means for a period the duration of which 27 corresponds to that of the operation of said motor means and at a rate which corresponds to the speed of operation of said motor means, utilizing the operation of said motor means for deriving a second alternating component of electromotive force the magnitude of which is a function of the speed of operation oisaid motor means and the phase of which corresponds to the direction of operation of said motor means, opposing said second component and said amplified component in the control of the energization of said motor means to suppress the increase in the rate of motion of said exhibiting member tending to be e!- iected by said motor means as said amplified component increases in magnitude, thereby to expand the range of variation of said first men- 'tioned electromotive force over which the rate exhibiting member in said predetermined position when the magnitude of said first mentioned electromotive force decreases below a predetermined value.

5. The method of positioning a movable member in substantially linear accordance with the duration and the amplitude of swing of a regularly fluctuating electromotiv force which consists of the steps of applying said electromotive force to motor means to control the energization of said motor means for operation of the latter for a period the duration of which corresponds to that of said electromotive force and at a speed which is in substantially linear accordance with the amplitude of swing of said electromotive force, utilizing the operation of said motor means for moving a-movable member from a predetermined position for a period the duration of which corresponds to that of the operation of said motor means and at a rate which corresponds to the speed of operation of said motor means, continuing such control of the energization of said motor means while the amplitude of swing of said electromotive force is greater than a predetermined value, and additionally controlling the energization of said motor means in accordance with the position of said movable member as required to restore and maintain said movable member in said predetermined position when the amplitude of swing of said electromotive force decreases below a predetermined value.

6. The method of positioning an exhibiting member in substantially linear accordance with I the duration and the magnitude of an electromotive force of one polarity or of opposite polarity and in one direction or the other depending upon said polarity which consists of the steps of translating said electromotive force into anelectromotive force having an alternating component of a magnitude and of one phase or of opposite phase determined respectively by the magnitude and polarit of the first mentioned electromotive force, amplifying said alternating component, applying said amplified component to motor means to control th energization of said motor means for operation of the latter in a direction corresponding to the phase of said amplified component for a period the duration of which corresponds to that of said amplified component and at a speed which is in substantially linear accordance with the magnitude of said amplified component, utilizing the operation 01' said motor means for moving an exhibiting member from a redetermined position in a direction corresponding to the direction or operation of said motor means for a period the duration of which corresponds to that of the operation of said motor means and at a rate which corresponds to the speed of operation or said motor means, utilizing the operation of said motor means for deriving a second alternating component of electromotive force the magnitude of which is a function of the speed of operation of said motor means and the phase of which corresponds to the direction of operation 01' said motor means,

opposing said second component and said amplified component in the control of the energization of said motor means to suppress the increas in the rate or motion or said exhibiting member tending to be eflected by said motor means as said amplified component increases in magnitude, thereby to expand the range of variation oi'ssaid first mentioned electromotive force over which the rate'of movement of said exhibiting member varies from zero to a maximum, and additionally controlling the energization oi said motor means in accordance with the position of said exhibiting member as required to restore and maintain said exhibiting member in said predetermined position when the magnitude of said first mentioned electromotive force decreases below a predetermined value.

'1. In apparatus to obtain a measure of the strength 01' a magnetic field and including an exploring device arranged to be moved relatively to the magnetic field to produce an electrical effect the magnitude of which is a function of the strength of said magnetic field and also the rate of relative movement of said magnetic field and exploring device, the combination comprising an exhibiting member to indicate the strength of said magnetic field, an electric motor to adjust said exhibiting means, electronic amplifying means having an input circuit to which the electrical effect produced by the exploring device is applied to control said electronic amplifying means, and having an output circuit directly electrically connected to said electric motor and arranged to energize said electric motor in such manner that said electric motor adjusts said exhibiting member for a, period in accordance with the duration of said electrical effect and at a rate in substantially linear accordance with the magnitude of said electrical effect, and means operative when said electrical efiect is less than a predetermined value and said exhibiting member is deflected from a predetermined position to control said control means as required to cause said electric motor to adjust -said exhibiting member to said predetermined position.

8. The combination of claim 7 wherein said last mentioned means includes a slidewire resistance and a relatively movable engaging contact connected in an electrical network and so related in position when said exhibiting member is in said predetermined position that no output electrical effect is obtained from said network. and mechanical means operated by said eelctrical motor to change the positional relation between said slidewire resistance and contact to produce an output electrical eflect from said network tending to control said control means when said exarrears 29 electrical efiects upon said control means and thereby upon said electric motor.

10. The combination of claim 'I wherein said control-means includes a detector circuit having impedance upon which said electrical effect is impressed, and means to vary the proportion of said electrical effect which is impressed on said detector circuit without significantly varying the impedance of said detector circuit.

11. The combination of claim "I wherein said control means includes a detector circuit having impedance upon which said electrical effect is impressed and wherein the means operative to control the control means to cause adjustment of the exhibiting member to said predetermined position when said electrical effect is less than a predetermined value includes a slidewire resistance and a relatively movable engaging contact connected in an electric network and so related in position when said exhibiting member is in said predetermined position that no output electrical efiect is obtained from said network, mechan-.

ical means operated by said electric motor to change the positional relation between said slidewire resistance and contact to produce an output electrical effect from said network which is impressed on said detector circuitandtends to con.- trol said control means, means to vary the proportion of said first mentioned electrical efiect which is impressed on said detector circuit without significantly varying the impedance of said detector circuit, adjustable meansto-apply an electrical effect on saiddet'ector circuit to substantially eliminate the influence of substantially constant extraneous electrical effects upon said control means and thereby upon said electric motor, and means to eliminate the output electrical effect from said network'to facilitate adjustment of said adjustable means.

12. In apparatus to obtaina measure of the. strength of a magnetic field and including an exploring device arranged to b moved relatively islessthanapredeterminedvalueandsaide'xr hibiting member is deflected from a predetermined position to control said control means as required to cause said electric motor to adjust said exhibiting member to said predetermined position.

18. In apparatus to obtain a measure of the strength of a magnetic field and including an exploring device arranged to be moved relatively to the magnetic field to produce an electromotive force the magnitude of which is a function of the strength of said magnetic field and also the rate of relative movement of said magnetic field and exploring device and the direction of which is dependent upon the polarity of said magnetic field, the combination comprising an exhibiting member to indicate the strength 'of said magnetic field, an electric motor to adjust said exhibiting means, electronic amplifying means having an input circuit to whichthe electromotive force produced by the exploring device is applied to control said electronic amplifying means. and having an output circuit directly electrically connected to said electric motor and arranged to energize said electric motor in such manner that said electric motor adjusts said exhibiting member for a period in accordance with the durationof said electromotive force and at a rate in substantially linear accordance with the magnitude of said electromotive force'and in a direction determined by the direction of said electromo tive force, means associated with said control means to produce a second electromotive force the magnitude of which is. a. function of the rate of adjustment of said exhibiting member by said electric motor. means to oppose said second electromotive force to said first mentioned electromotive force to suppress increases 'inthe rate r of adjustment of said exhibiting member by said to the magnetic field to produce-an electrical efg feet the magnitude of which is a function of the strength of said magnetic field and also the rate of relative movement of'said magnetic field and exploring device, the combination comprising an exhibiting member to indicate the strength of said magnetic field, an electric motor to adjust said exhibiting means, electronic amplifying 5 means having an input circuit to which the elec- .trical effect produced by the exploring" device is applied to control said electronic amplifying electric motor as said first mentioned electromotive force increases in magnitude to thereby expand the range of variation of said first mentioned electromotive force over which the rate of adjustment of said exhibiting member by said electric motor varies from standstill to full speed,

- and means operative when. said first mentioned means, and having an output circuit directly electrically connected to said electric motor and arranged to energize said. electric motor in such manner that said electric motor adjusts said ex hiblting member for a period in accordanc with the duration of said electrical effect and at a rate in substantially linear accordance with the magnitude of said electrical effect, means associated with said control means .to produce a second electrical effect the magnitude of which is a function of the rate of adjustment of said exhibiting member by said electric motor, means control Said co ol means.

electromotive force is less than a predetermined value and said exhibiting element is deflected from a predetermined position to control said control means as required to cause said electric motor to adjust s'aid exhibiting member to said predetermined position. Q a I '14; The combination of claim 13 whereinsald last mentioned means includes-means responsive to the extent and direction "of departure of said exhibiting member from said predetermined position over a limited range to produce an elec- 'tromotive force which is variable from zeroto a maximum as said exhibiting member is moved through said limited range and is of one polarity .or the other accordingly as said exhibiting member is deflected in one. direction or the other from said predetermined position, and means to v apply said last mentioned electromotive force to to oppose said second electrical effect to said first mentionedelectrical effect to suppress increases in the rate of adjustment of said exhibiting member by said electric motor as said first mentioned electrical effect increases in magnitude to thereby expand the range of variation of said first mentioned electrical effect over which the rate of adjustment of said exhibiting member by said electric motor varies from standstill to full speed.

15. The combination of claim 13 wherein said control means includes a detector circuit having impedanc upon which said first mentioned electromotive force is impressed and wherein the means operative to control the control means to cause adjustment of the exhibiting member to said predetermined position when said first mentioned electromotive force is less than a predetermined value includes a slidewire resistance and means operative when said electrical eifect' 76 and 'a relatively movable engaging contact connected in an electric 31 network and so related in position when said exhibiting member is in said predetermined position that no output electromotive'iorce is obtained from said network, control switch means arranged to be actuated when said exhibiting member is deflected from said predetermined position, operated y said electric thereby upon said electric motor, and means to eliminate the output electromotive force from said network to facilitate adjustment of said adjustable means. 1

16. In apparatus to obtain a measure of the strength 01 a magnetic field comprising an exploringdevice arranged to be moved relatively to the'magnetic field to produce anelectromotive force the magnitude of which is a .iunction oi the strength of said magnetic field and also the rate 01 relative movement of said magnetic field and exploring device the combination comprising,

means to derive, from said electromotive force a corresponding electromotive, force alternating in polarity at a. fixed frequency, means to amplify said derived electromotive force, an exhibiting member to indicate the strength oi said magnetic field, an electric motor to adjustsaid exhibiting means, means under control of the amplified quantity of said derived electromotive force to control said electric motor in such, manner that said electric motor adjusts said exhibiting mem electrical jefi'ect to a,sss,41a

be connected to a source oi' alternating current and having acommon output circuit to which said electronic device ar connected inparallel l n. m nuw vrr w fl l ose electronicdevicestoaeiectivelyrenderoneoisaid devicesmbre oondnctivethan the other to an extent dependent upon the magnitude or said electrical elect, and an electric motor, having a windingadaptedtobeconnectedtothealternating currmt supply-source and a winding connected to, the output circuit or said electronic devices to adiust mid exhibiting member, said grid suppl means, including a network in which said second mentioned winding is connected to derive an electrical eilect proportional to the motor speed and means to oppose said derived said first mentioned electrical eilect.

18; In apparatus to'obtain a measure or the strength ota magnetic field and including an exploring device arranged to be moved relatively to the magnetic field to produce-an electrical elect the magnitmie oil-which is a function of the strength 01' field and also the rate oi relative movementoi said magnetic field and exploring device, the combination including an i exhibiting member to indicate the strength'oi'.

said magnetic field, a pair of grid controlled elec her for a period in accordance with the dura- 'tion of said electromotive force and at a rate in substantially linear accordance with the magni- .tude of said derived electromotive force, means associated with said control means to derive from the amplified quantity of said derived electromotive iorce an electromotive iorce alternating in polarity at said fixed frequency and the magnitude oi which is a function of the rate of ad- 'iustment oi said exhibiting member by said electric motor, and means to oppose said last mentioned electromotive iorce to the amplified quannitude to thereby expand the range of variation of said first mentioned electromotive force over which the .rate of adjustment of said exhibiting member by said electric motorvaries from standstill to full speed."

l'lylnapparatus'to obtain a measure'oi the strengthoi' a magnetic field and including an exploring device arrangedto be moved relatively to the magnetic field to produce an electrical effeet the magnitude of which is a. function of the strength 01' said magnetic field and alsothe rate of relative movement of said magnetic field and exploring device, the combination'including an exhibiting member to indicate the strength of said magnetic field, a pair of grid controlled electronic devioes having output circuits adapted to tronic devices having output circuits adapted to be connected to a source 01 altcrna current and having a commonoutput circuit to which said electronic devices are connected in parallel relation, means to supply-grid potentials to said electronic devices 'to selectively' render one of said devices more conductive than the other to an extent dependent upon'the magnitude of said electrical efiect, and an electric motor having, a winding adapted to be connected to the alternating current supply source and a winding connected to the Output circuit oi said electronic devices to adjust said exhibiting member. said grid supply means including an electric generator driven by said electric motor to derive an electrical enact proportional to the motor speed and means to oppose said derived electrical eiiect to said first mentioned electrical ed'ect. r

19. In apparatlnto obtain a measure oi the strength of amagnetic field and including an exploring device arranged to be moved relatively to themagneflcfield to produce anelectricaleflect the magnitude or which is a function of the strength of said magnetic field and also the rate 01' relative movement o! said magnetic field and exploring a device, the combination including an exhibiting member to indicate the strength 01' said magnetic field, a pair of grid controlled electronic devices having output circuits adapted to be connected to a source or alternaflng current and having a common output circuit to which said electronic devices .are connected in parallel relation, means includthan the other to an extent dependent upon the magnitude-of said electrical efiect, and an electric motor having a winding adapted to be connecied to the alternating current supply source and a winding connected to the output circuit of said electronic devices to adjust said exhibiting member, said grid pply means including a feed-- back circuit between two or the stages oi said amplifier-ham connected therein a pair of oppositely connected rectlfiers the resistancevalues 

